The present invention generally relates to high pressure seals and in particular to a high pressure seal used in subsea drilling riser systems.
Sub-sea riser systems are commonly designed as a series of drawn steel pipes joined together by a pin and box arrangement at each end to form a “string”. The riser string joints are screwed together and each joint has an internal rubber seal between the pin and box in a specially machined housing. The purpose of the seal is to prevent the loss of the pressurized fluids transported through the string from the well head to the surface, and in the event of a loss of internal pressure within the string, to keep seawater from entering the string.
There are several designs of seal system and commonly this has been a molded rubber D section with embedded tightly coiled wire known as a garter spring on either side of the seal to prevent extrusion of the rubber from its housing under the pressure of the well fluid. These seals are commonly called S-seals in the industry. Current technology has the capability of the seal to withstand extrusion up to 15,000 psi operating pressure and 22,500 psi test pressure. This capability is maintained over a temperature range of −18° C. to 121° C. Pressures and temperatures exceeding these limits cause the rubber to extrude past the spring resulting in a leak or blowout.
Glands housing these seals are known as “closed glands” which means that a seal must be compressed and contorted or stretched into the gland. An “open gland” seal is able to slide into position and then axially retained without the need for contortion or stretching. Open gland assemblies can generally withstand higher pressures because anti-extrusion rings, sometimes referred to as back-up rings, can be made from rigid material without the need for a split or break. Conventional closed gland sealing arrangements require anti-extrusion material to be elastic enough to bend and stretch or require a split in a ring creating a path for extrusion. They are typically multi-piece designs making assembly complicated.
Deeper water systems now require the riser string to operate at 20,000 psi and be capable of test at 30,000 psi, with a temperature profile of −29° C. to 150° C. using a closed gland. There is currently no corresponding unitized riser seal capable of withstanding this pressure and temperature in the application.